1. Field of the Invention
This invention relates to pneumatic radial tires for airplanes used at high speed under high loading, and more particularly to a pneumatic radial tire for airplanes having improved service durability and resistance to uneven wear by the improvement of the belt structure.
2. Relates Art Statement
Since radial or semi-radial tires for airplanes are particularly used at high speed, they comprise a belt composed of plural belt layers each containing cords arranged substantially in parallel to an equator of the tire (hereinafter represented as a circumferential direction) from a viewpoint of the resistance to the standing wave phenomenon. This belt structure is advantageous in the weight reduction of tire because the pressure test required for the airplane tire (no occurrence of bust under a pressure corresponding to 4 times of service internal pressure) can be satisfied by the minimum number of belt layers, which is useful in view of the weight and low heat buildup exerting on the service cost of the airplane. In this belt structure, however, cords for the belt layer are arranged in the circumferential direction, so that they hardly extend or contract during the running of the tire. Furthermore, since the tire diameter is smaller in the tread shoulder than in the tread center, the tread shoulder portion is dragged during the running of the tire to cause uneven wear. In order to solve this problem, therefore, the following belt structure has been proposed.
That is, such a belt structure is a combination of circumferential belt layer containing cords arranged in parallel to the equator of the tire and a cross belt layer containing cords crossed with the equator. This is disclosed in U.S. Pat. No. 4,402,356.
In this belt structure, as the number of cross belt layers increases as compared with the number of circumferential belt layers, the occurrence of uneven wear can be reduced. However, the cross belt layer is low in circumferential rigidity as compared with the circumferential belt layer, so that the total number of belt layers necessarily increases for satisfying the pressure test, which is disadvantageous in view of the tire weight. And also, the total thickness of the belt structure undesirably increases, resulting in the increase of heat buildup to lower the service durability against separation failure of the belt.